This invention relates to electrical resistors and in particular to an electrical insulator operating at high temperature, the insulator having an alumina crystal with a niobium face sheet bonded thereto and being subjected to the effects of molten lithium.
The present invention was made in connection with the development of a thermoelectric power generator for use in space vehicles. In general, the hot side of a thermoelectric cell is attached to the wall (made of Nb or Nb-1% Zr) of a heat exchanger through which molten lithium circulates in a closed loop from a nuclear reactor. The cold side of the thermoelectric cell is attached to the wall (also of Nb or Nb-1% Zr) of another heat exchanger through which liquid lithium circulates in a second closed loop to a radiator that rejects heat into space. Lithium is used in the heat exchangers because of its favorable mass, specific heat and viscosity. Its frozen condition during launch provides safety advantages. After insertion into the required orbit, the lithium distributed through the system is thawed by a heat pipe network that derives its heat from the reactor.
The nominal hot side temperature of the thermoelectric cell is approximately 1350K, and the cold side temperature is approximately 870K. Typically, the cell will develop a 100 volt potential.
The thermoelectric module of the cell must be insulated electrically from the heat exchangers which are at spacecraft ground potential, yet must be in good thermal contact with them. Accordingly, an insulator material must be placed between the thermoelectric module of the cell and the heat exchanger which has the somewhat contradictory properties of high electrical resistivity and a sufficiently high thermal conductivity so as not to represent a significant parasitic temperature drop from the heat exchanger to the thermoelectric module. Furthermore, these properties must remain stable for at least the design life, seven years, in a fairly hostile environment, at high vacuum, in temperatures up to 1350K and with voltage drops of up to 100 V applied continuously across the insulator.
In the development of a suitable insulator that would meet the above requirements, four ceramic materials were identified as having suitable promise: polycrystalline Al.sub.2 O.sub.3, BeO, BN and single crystal alumina (SxAl.sub.2 O.sub.3, sapphire). Of the polycrystalline materials, only BeO and BN continued to show promise. Polycrystalline Al.sub.2 O.sub.3, when tested under the proposed operating conditions, demonstrated catastrophic breakdown of insulation resistance at relatively short lifetimes. BN has a low coefficient of thermal expansion, which precludes it being bonded to Nb or Nb-1% Zr. Single crystal alumina was found to be the best material. It exhibited more than sufficient electrical resistivity. Its stability with respect to either electrochemical or thermochemical degradation as judged from accelerated tests at 1573K was excellent and appeared to be adequate to enable it to serve for more than the required seven years.
The single crystal alumina insulators had a face sheet of niobium (Nb) diffusion bonded thereto, such face sheet being needed to enable the insulators to be physically attached to the wall of the heat exchanger and to provide good thermal conduction between the heat exchanger and the alumina crystal. Niobium foil was used for the face sheet because niobium can be easily and strongly bonded to sapphire and has a coefficient of thermal expansion that is very close to that of sapphire.
However, the evaluations of the single crystal alumina insulators were carried out in the absence of lithium (Li), and there was some suspicion that thermochemical degradation might occur faster when lithium was present than when it was absent. In particular, J.D. Selle and J.H. DeVan had reported in "The Reduction of Al.sub.2 O.sub.3 in Niobium-Lithium Systems at 1000.degree. C.," Oak Ridge National Laboratory report ORNL/TM-5828; Chem.Abst. 88:139, 927t (1977), published July 1977, the formation, at temperatures of the order of 1000.degree. C. or higher, of Nb.sub.x Al.sub.y intermetallics at the Al.sub.2 O.sub.3 //Nb interfaces in Al.sub.2 O.sub.3 sandwich structures, where a thin layer of Nb separates Al.sub.2 O.sub.3 from liquid Li. The same effect occurs if the Nb is replaced by a Nb alloy such as Nb-1% Zr.
Tests were then performed on insulator specimens in which the Al.sub.2 O.sub.3 was a sapphire single crystal incorporated in either a four-layer sandwich structure, Al.sub.2 O.sub.3 //Nb//Nb-1% Zr//Li, with 0.13 mm thick Nb foil and 0.75 mm thick Nb-1% Zr layer, or in a five-layer sandwich, Al.sub.2 O.sub.3 //Nb//V//Nb-1% Zr//Li, which also included a 0.05 mm thick layer of vanadium. Tests were carried out, some at 500 hours and some at 1000 hours, at temperatures of 1373K, 1474K, and 1573K. The Selle-DeVan Nb.sub.x Al.sub.y layers were observed in the Nb layer in all specimens.
These intermetallic layers are very undesirable in an insulator for the present purposes, because the layers have a high thermal resistance, and the brittleness of the layers can result in fractures along the layers with consequent mechanical failure of the insulator.